In an effort combat climate change, scientists are researching ways to reduce the amount of sunlight reaching the earth. The reasoning behind the study is that these temporary sunlight reduction methods have the potential to reduce temperatures and therefore reduce warming.

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A new computer analysis of future climate change that considers emissions reductions together with sunlight reduction reveals that cooling the earth would only be necessary if the planet is found to heat up easily with added greenhouse gases.

Researchers at the Department of Energy's Pacific Northwest National Laboratory explored sunlight reduction methods, or solar radiation management, in a computer model that followed emissions' effect on climate. The analysis shows there is a fundamental connection between the need for emissions reductions and the potential need for some sort of solar dimming.

"It's a what-if scenario analysis," said Steven Smith with the Joint Global Change Research Institute in College Park, Maryland, a joint venture between PNNL and the University of Maryland. "The conditions under which policymakers might want to manage the amount of sun reaching earth depends on how sensitive the climate is to atmospheric greenhouse gases, and we just don't know that yet."

But how would scientists reduce the amount of sunlight? Could we construct a giant earth umbrella, or concoct some type global sunscreen? The researchers are contemplating shading the earth from the sun's heat by either brightening clouds, mimicking the atmospheric cooling from volcanic eruptions or putting mirrors in space.

How much sun blocking might be needed depends on climate sensitivity. Scientists measure climate sensitivity by how many degrees the atmosphere warms up if the concentration of carbon dioxide doubles.

According to a recent NOAA's report, the earth's temperature has risen about 0.62 degrees Celsius (1.12 degrees Fahrenheit) since the beginning of the 20th century as the carbon dioxide in the atmosphere has grown from 290 parts per million to 379 parts per million.

If humans were to stop putting more carbon dioxide into the air, the climate would still continue to change until it reaches equilibrium. However, because we do not know how sensitive the planet is to greenhouse gases, we do not know at what temperature the earth will reach equilibrium.

The analysis might help future policymakers plan for a changing climate. The study can also help decision-makers evaluate solar reduction technologies.